Known power tools have been configured such that a battery pack, which constitutes a drive-power supply of a tool main body, is detachable. The battery pack comprises a case (housing), which constitutes an outer shell, and a battery main body, which is installed in an interior of the case. The battery main body comprises a plurality of rechargeable battery cells. The battery cells that constitute such a battery main body are lined up and disposed so as to minimize the required space in the interior of the case. Specifically, the battery cells are disposed in parallel and the longitudinal ends (plus and minus electrodes) of the battery cells are aligned and arranged (oriented) in an alternating manner. With such a parallel arrangement, the battery pack is arranged transversely such that the longitudinal direction of the battery cells is oriented in a direction orthogonal to the direction in which the battery pack is slid when mounted on a power tool (see e.g., Japanese Laid-open Patent Publication 2003-142051 and its US counterpart US 2013/0230757).
When the remaining charge of the battery pack becomes low, the battery pack is removed from the tool main body and is charged by a dedicated charger. Due to the internal resistance of the battery cells and the charging current, the battery cells generate heat while being charged. Consequently, in the above-noted known battery pack system, the dedicated charger is provided with a ventilation mechanism that forces a cooling draft (cooling air) over the battery cells to cool them during the charging. That is, when the dedicated charger is charging the battery pack, the ventilation mechanism delivers a cooling draft (forced cooling air) into the interior of the case. This cooling draft flows through the interior of the case in a direction parallel to (along) the mounting direction of the battery pack. As a result, because the cooling draft blows transversely against each of the battery cells that are disposed in parallel, the cooling draft can cool every one of the battery cells with little unevenness.